The use of liquid polymers in polyblend compositions has been tried in the past, typically however using liquid polymers derived from a single monomer. The incorporation of these liquid polymers typically decreased the physical properties of the resulting composition. This invention capitalizes on the use of a liquid block copolymer which comprises both isoprene and butadiene in an elastomeric composition. This elastomeric composition is useful in for example, the preparation of tires, having at least a portion which contains the liquid block copolymer.
Rubbers are typically compounded with numerous chemical agents prior to being molded and cured into desired articles of manufacture. The rubber compounding procedure utilized is normally carried out in a mixing device which relies on shearing forces, such as a Banbury mixer or a mill mixer. During this compounding procedure, the rubber is commonly mixed with sulfur, accelerators, carbon black, antidegradants, and other desired rubber chemicals. It is also very common to blend more than one type of rubber in the compounding procedure.
The high shearing forces required to attain homogeneous mixtures result in a degradation of the rubber. For this reason, the Mooney viscosity of the rubber being compounded decreases during the mixing procedure. This problem becomes more serious in rubber compounds which contain reinforcing materials, such as carbon black, which are included to increase the modulus of rubber. It is known that rubber compounds which contain large amounts of carbon black are particularly difficult to process. Nevertheless, it is often necessary to include significant amounts of carbon black in rubber compounds to attain the desired modulus.
Processing oils are known to improve the processability of rubbers. However, the inclusion of processing oils in such rubbers results in a decrease in modulus. Accordingly, the inclusion of substantial amounts of processing oils in rubber compounds is frequently not a viable option for attaining good processability.
High rubber performance requirements have traditionally necessitated compounding rubbers which have very poor processability characteristics. It has been known to add a liquid polymer covulcanizable with a rubbery polymer for the purpose of obtaining a rubber composition which has improved processability and can give vulcanizates having improved tensile properties. The resulting rubber composition, however, has the defect in that vulcanizates from it have degraded dynamic properties, for example, reduced rebound resilience. It is also known to add lubricants such as mineral oil and waxes to improve the elongation at fail of such polyblends. However, such additives lower the tensile strength and heat distortion temperature and have not improved overall toughness.
What has not, however, been taught in the prior art, is a way to incorporate liquid block copolymers into vulcanizable rubbers without a corresponding decrease in the physical properties of the rubber.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber elastomer". In the description of this invention, the terms "rubber" and "elastomer" can be used interchangeably, unless otherwise distinguished. The terms "rubber composition", "compounded rubber" and "rubber compound" can be used interchangeably to refer to "rubber which has been blended or mixed with various ingredients and materials" and such terms are well known to those having skill in the rubber mixing or rubber compounding art.